1. Field of the Invention
The present invention relates to an optical disc apparatus including a CD-ROM, a CD-R, a CD-RW drive, a DVD drive, a Blu-ray disc drive and such, and a controlling method thereof.
2. Description of the Related Art(s)
Recently, types of optical disc medium have become diversified. All optical disc media commonly have a structure of plural layers and comprise a signal surface, on an inner side of a protection layer provided on a surface side of the medium, on which a signal is recorded. On the other hand, many parameters such as the thickness of a disc medium, a distance from the surface to the signal surface, the number of signal surfaces (a DVD, for example, includes a maximum of two signal surface layers) and a wavelength of a laser beam to be used for reading information from the signal surface, are different.
Accordingly, a dedicated drive for each optical disc medium has been typically used. Users, however, should be experienced in operating the respective drives and an economic burden is heavy since the users should purchase and install a dedicated drive for each optical disc medium. This has created a need for a drive (an optical disc apparatus) capable of handling plural types of optical disc medium. For such a drive, technology has been developed that takes advantage of a difference in wavelength of a light source (a laser) used for reading in accordance with the type of an optical disc medium, with one objective lens being used for changing the number of the lens aperture by means of an optical element having a wavelength selective characteristic.
An optical pickup for such a drive comprises a light-emitting element 11 for outputting plural wavelengths of laser beam, a beam splitter 12, a photodetector 13 and an objective lens body 14, as shown in FIG. 5. The objective lens body 14 comprises an objective lens 14L and a hologram element 14H including a diffraction grating.
The light-emitting element 11 is a semiconductor laser element for outputting three wavelengths of laser beam different from each other (a so-called three-wavelength laser), for example. In the case that the three wavelengths are arranged to correspond to a Blu-ray disc, a DVD (digital versatile disc) and a CD (compact disc), for example, control is carried out so as to output a laser beam having a wavelength of 405 nanometers for Blu-ray disc, 650 nanometers for a DVD and 780 nanometers for a CD.
The beam splitter 12 leads light outputted from the light-emitting element 11 to an objective lens body 14 side. The beam splitter 12 also leads light, which is reflected on a medium and inputted through the objective lens body 14, to a photodetector 13 side. The photodetector 13 comprises plural optical detecting elements arranged in an N×N matrix, for example. The photodetector 13 comprises a cylindrical lens, for example, for measuring a beam diameter. The light led by means of the beam splitter 12 reaches each of plural light-receiving elements through the cylindrical lens. The photodetector 13 respectively outputs signals for the strength of the light detected by the respective plural light-receiving elements.
The hologram element 14H of the objective lens body 14 refracts the laser beam, which is led through the objective lens body 14 and reflected on a medium, so that the laser beam has a numerical aperture (NA) corresponding to the wavelength thereof to lead the laser beam to the beam splitter 12. The objective lens 14L is an aspherical lens. The objective lens 14L refracts and outputs the laser beam, which is led from the light-emitting element 11 through the beam splitter 12 and the hologram element 14, so as to be focused on a point at a distance of a predetermined focal length F different for every wavelength. The objective lens 14L also concentrates the laser beam reflected on a medium to lead the laser beam to the hologram element 14H.
A signal indicating a difference in focus of the laser beam from a recording surface of an optical disc medium (a focusing error signal; an FE signal) and a signal representing the total strength of light having reached the light-receiving element (a pull-in signal; a PI signal) are generated from the signal outputted from the photodetector 13 (an RF signal). Usually, there is also a signal indicating a tracking error (a TE signal), generated from the signal outputted from the photodetector 13. Detailed description thereof will be omitted here.
The FE signal is a signal such as a signal shown in FIG. 6A. That is, the FE signal is substantially “0” in an in-focus condition. The FE signal has positive and negative peaks, respectively, when the objective lens body 14 is at a predetermined distance from an in-focus point in the case of varying a distance between the optical disc medium and the objective lens body 14 with the in-focus point being the center. The wavelength of the signal is referred to as a focusing error wavelength, hereinafter.
The PI signal is a signal such as a signal shown in FIG. 6B. That is to say, the PI signal has a peak at the in-focus point. FIGS. 6A and 6B schematically illustrate the FE signal and the PI signal, respectively.
Using the optical pickup to control a distance between the objective lens body 14 and a surface of a medium so that a distance from the objective lens 14L to the signal surface inside the medium becomes the focal length F, namely, so that focusing on the signal surface is possible, allows signals to be read out corresponding to plural optical disc media. The FE signal and/or the PI signal are/is used for judging whether the condition is in focus or not. For example, it is judged to be in focus when an absolute value of the FE signal becomes lower than a predetermined threshold value (approaches “0”) after surpassing the peak. It is also judged to be in focus when the PI signal exceeds a predetermined threshold value.
JP-A-2002-269770 (which corresponds to US2002/136103) discloses a technology for appropriately controlling an arrangement relationship between an objective lens and a medium in an optical disc medium having two or more signal surface layers.
When the conventional optical pickup is used so as to correspond to a large number of optical disc media, however, unnecessary light in the objective lens body 14, which is not required in changing the numerical aperture, reflects inside the objective lens body 14 as reflection light on the surface of an optical disc medium or the signal surface before detecting reflection light on the signal surface in moving the objective lens body 14 to the optical disc medium, so that at least one fake signal (FAKE) is detected in some cases.
In accordance with the kind and/or the characteristics of an optical disc medium to be a subject, the shapes of the FE signal and the PI signal, which concern the reflection light on the signal surface, are likely to resemble those of the fake signals of the FE and PI signals to the extent that a distinction between the FE and PI signals and the fake signals is difficult. For example, in the case where a cause of Fake is reflection on the surface in a media having low reflectance (such as a CD-RW), the strength of the reflection light on the signal surface is sometimes smaller than that of Fake.